In order to evaluate the role of mucoid coating of clinical isolates of Pseudomonas aeruginosa as a virulence factor, opsonophagocytosis of mucoid and nonmucoid strains of P. aeruginosa were studied by three different methods: uptake of [3H]adenine-labeled bacteria, oxygen consumption, and chemiluminescence production by phagocytosing polymorphonuclear leukocytes. All three methods showed a strong correlation with regard to phagocytosis of the bacterial isolates. As a group, the mucoid strains of P. aeruginosa demonstrated significantly reduced phagocytic uptake by leukocytes when compared to nonmucoid strains, although occasionally mucoid strains did exhibit normal uptake. In vitro growth of mucoid strains under nonstationary conditions was associated with reduced mucoid coating and with a corresponding increase in susceptibility to leukocyte phagocytosis. Although the overall bactericidal activity of human polymorphonuclear leukocytes for mucoid strains was diminished, this appears to be a function of reduced ingestion since the rate of intracellular killing was similar for mucoid and nonmucoid strains. The enhanced susceptibility of mucoid strains to spontaneous bactericidal activity of pooled human serum was confirmed. Mucoid-producing strains produced less protease in vitro; however, the role of this phenomenon in influencing phagocytosis is unknown.